Roll 1 piece of apple into each crescent roll. Place in 13x9 baking dish which is lightly sprayed with cooking spray. Make 2 rows in middle of pan, 8 in each. Push dumplings close together in middle, leaving space at sides of pan. Mix together sugar, butter, cinnamon and vanilla and pour over dumplings in pan. Pour the Mountain Dew around edges of pan, not directly onto the dumplings. Bake uncovered at 350°F for 30 minutes.